1. Field of the Invention
This invention relates to the art of injection molding plastic products and particularly to the heating and cooling the skin of the mold cavity to achieve an improved surface finish.
2. Description of the Prior Art
In the manufacture of molded plastic parts many applications require that the part have a shiney or glossy surface when the part comes out of the mold. In the injection molding of solid parts this shiney surface is achieved. However, in the manufacture of foam molded parts, the surface finish is usually a mottled finish with swirls because the gas used or generated in the process of foam molding escapes to the surface of the part in the mold until the surface starts to solidify. As the surface solidifies, gas is trapped inside the part and the cellular inner structure develops. During the cooling cycle, while gas is still escaping to the surface, the mottled finish and swirls occur. This dull finish is objectionable because the part manufactured by this foam molding process now requires a secondary operation of finishing the parts since the mottled appearance of the original finish is not commercially acceptable.
A method of eliminating the mottled appearance is to retain the heat above the deformation temperature of the resin at the mold surfaces of the part until the cavity is full. This is a difficult and essentially an impractical approach after the part has been molded. Other methods are discussed in an article in Plastic Technology, Vol. 22, No. 5, May, 1976, pages 33-36, to heat up the mold itself both before and during the molding cycle. This article details lengthening the mold fill time to get a no slip flow condition at the surface of the mold during filling. Another is to increase the melt temperature to again reduce the slip/no slip condition and the third is to increase the mold temperature which causes a thin surface layer of the injected melt to retain heat which reduces the slip/no slip condition.
A process of heating the mold is described in U.S. Pat. No. 3,044,118 which heats up the whole mold above the melting temperature of the plastic, and then cools the mold down after the injection cycle using carbon dioxide gas. This method is very slow and time consuming depending on the particular part being molded and the structural sections within the part. Also the mold is subjected to extraordinary stress during this heating and cooling cycle and such stress could crack or weaken the mold which limits its useful life. Other solutions are to use low mass molds and incorporate low-mass-interior conducting surfaces, backed-up by insulators to allow the mold to be heated by the heat of the polymer. This of course requires a whole new design approach to mold making itself.
A most recent method disclosed in the process of running superheated steam through the water lines in an existing mold and then, after the injection cycle is complete, pass chilled water through the mold to cool off and solidify the part so it can be removed from the mold. The manufacturing cycle time to make the part by this method is increased from 25% to 50% over normal time in order to retard the heat loss of the resin at the mold surfaces and give the nice shiney appearance which is desired. Although giving the desired finish, the cycle time is generally considered too long for the benefit received.
An object of my invention is to heat the working surface or skin cavity of the mold with a condensing vapor by injecting the vapor directly inside the mold cavity to come in contact with the skin of the mold after it is closed and quickly evacuate it of all vapors and condensate just prior to the injection cycle. Another object of the invention is to pass cooling fluid through the interior sections of the mold to cool the surface of the part from the inside out to reduce the cycle time and produce a shiney surface.
Another object of the invention is to use poppet type valves opening directly into the mold cavity for inputting the condensing vapor for heating up the mold surface and a similar valve for evacuation of the cavity. It is further an object of this invention to heat the surface of the mold to a temperature that exceeds the heat distortion point of the material being molded.
It is a further object of this invention to seal the periphery of the cavity of the mold with a seal to act as a pressure barrier in the mold to contain the condensing vapor.
It is also an object of this invention to measure the temperature of the skin or surface of the mold cavity during both the heating and cooling cycle to determine the appropriate time to inject the plastic and the proper time to open the mold after the cooling cycle.